Fluoroelastomers and in particular perfluoroelastomers such as those described in “Modern Fluoropolyrners”, edited by John Scheirs, Wiley Science 1997, offer excellent protection against high service temperatures and are resistant to a wide variety of chemical reagents. Fluoroelastomers have been used successfully in a number of applications due to their ability to withstand high temperatures and aggressive chemicals, as well as the ability of the fluoroelastomer gum to be processed using standard elastomer processing equipment. For example, fluoroelastomers have been used in fuel management systems such as automotive fuel hoses, filler neck hoses, injector o-rings, and the like. Fuel management applications require low fuel vapor permeation in combination with good low temperature properties, sealability, and flexural properties. Still further, fluoroelastomers have been used in the semiconductor industry in the chip manufacturing process where the fluoroelastomer may be used as seals in chip fabrication equipment. During chip manufacturing, the fluoroelastomer can be exposed to high temperature and aggressive chemicals. Still further, fluoroelastomers are being used as electrical cable insulators.
Fluoroelastomers are elastomers prepared by curing a fluoroelastomer precursor (“fluoroelastomer gum”) made from monomers containing one or more atoms of fluorine, or copolymers of such monomers with other monomers, the fluoromonomer(s) being present in the greatest amount by mass. The fluoroelastomer precursor is a fluoropolymer that is suitable to prepare a fluoroelastomer having desired elasticity properties. Typically, the fluoroelastomer precursor is an amorphous fluoropolymer or a fluoropolymer that hardly shows a melting point. When the fluoropolymer has a perfluorinated backbone, a perfluoroelastomer results but also polymers having a partially fluorinated backbone are used.
A commonly used process for the preparation of fluoropolymers is the aqueous emulsion polymerization which offers an environmental advantage over polymerization in solvents. Generally, the aqueous emulsion polymerization of fluorinated monomers is carried out in the presence of a fluorinated surfactant although techniques have also been developed in which no fluorinated surfactant is added to the polymerization system.
The art is further deplete with various modifications of the aqueous emulsion polymerization process to improve certain aspects thereof or to achieve particular objectives. For example, it is generally recognized in the art to pre-emulsify one or more fluorinated monomers.
It has also been suggested in the art to use micro-emulsions in the aqueous emulsion polymerization of fluorinated monomers. Micro-emulsions are stable isotropic mixtures of oil, water, and surfactant which form spontaneously upon contact of the ingredients. Other components, such as salt or co-surfactant (an alcohol, amine, or other amphiphilic molecule) may also be part of the micro-emulsion formulation. The oil and water reside in distinct domains separated by an interfacial layer rich in surfactant. Because the domains of oil or water are small, micro-emulsions appear visually transparent or translucent. Unlike emulsions and the pre-emulsions disclosed in the above references, micro-emulsions are equilibrium phases.
Fluoropolymer produced through aqueous emulsion polymerization may be cured to obtain the fluoroelastomer if the fluoropolymer contains so-called cure-sites that participate in a cure reaction to form a three dimensional network. A well-known cure reaction used to vulcanize the fluoropolymer involves the use of a peroxide whereby the fluoropolymer contains halogens, e.g. bromine or iodine that are capable of participating in a peroxide cure reaction. These halogens are typically introduced in the fluoropolymer by copolymerizing one or more fluorinated monomers with a fluorinated monomer that contains such a halogen.
Alternatively, the fluoropolymer may have one or more units that derive from a fluorinated monomer that has a nitrile group. Such a nitrile group can be used to cure the fluoropolymer in the presence of ammonia-generating compounds or other curatives or catalysts capable of causing curing of the nitrile groups.
It has now been found that when fluoropolymers having aforementioned cure-sites deriving from corresponding monomers having one or more cure-sites, are prepared through aqueous emulsion polymerization, the resulting fluoroelastomer may show a shiny wet looking surface after curing of the fluoropolymer. Additionally, the amount of organic material that can be extracted from the fluoroelastomer may be undesirably high. The presence of a shiny wet looking surface as well as a large amount of extractable organic material may cause the fluoroelastomer to be unsuitable for certain applications, in particular in demanding applications. This effect is particularly noticeable when low levels of the cure-site monomer are being used, e.g. 1 mole % or less.
Accordingly, there continues to be a need to improve the properties of fluoroelastomers and in particular, to improve properties of fluoroelastomers produced from fluoropolymers that are made through the aqueous emulsion polymerization process.